Expand make variables in vars about rules_jsonnet HOT 7 CLOSED

@mariusgrigoriu Just started out with using jsonnet for kubernetes manifests and we would have similar requirements in regard to ingress host names. I am a fan of having everything checked into the repository so my idea was making use of --code-file/file (not implemented here yet but see google/jsonnet#110 for usage) and then define it as something like:

jsonnet_to_json(
    name = "k8s",
    src = "k8s.jsonnet",
    outs = ["k8s.json"],
    file = select({
    "//common/config:namespace_dev": {"fqdn": ":fqdn_dev.txt"},
    "//common/config:namespace_something": {"fqdn": ":fqdn_something.txt"},
    }),
)

So one would run e.g. bazel build :k8s --define namespace=dev and that would then make it select the right checked in file which contains the fqdn.

Your approach is of course total valid as well and my thought of approach has it's own drawbacks but since I just thought of that yesterday/today and wanted to look into it in the near future I thought I'd mention it here.

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I think that approach is better if you have a limited number of environments. In our case, the name is dynamically generated from earlier in the build process. Each pipeline run spins up a whole cluster with a unique name. Normally I'd use stamp variables for something like that, but I'm integrating with a complicated make system while incrementally converting to Bazel. (I'm also new to Bazel so I'm sure there are better ways of accomplishing this that I don't know about.)

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Is this backwards incompatible for people who were previously using $ in that string and expecting it to be passed through to Jsonnet?

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A $ alone should be fine, however $( ) would be interpreted as a make variable. People who have those would need to use $$( )

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Are strings usually interpolated like that in Bazel? If not is it usually documented which ones have it / don't have it, and is there any kind of standard about that?

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Only certain arguments are substituted like that. The convention is to document when it happens. The most common example is the genrule: https://docs.bazel.build/versions/master/be/general.html#genrule.cmd

It seems to me that if jsonnet takes variables on the command line, then it would be handy to extend that type of capability all the way out by allowing variables to be passed in from the bazel command line.

That said, for my uses, I'm starting to generate a .libsonnet with variables that will be imported into various templates, but this feature might still be handy.

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Merged in #40
Thank you.

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